It is well recognized in the field of data storage that it is desirable to increase the storage density and reduce the cost of storage in information storage devices. This is generally true for all types of information storage devices, such as magnetic hard drives, optical drives, RAM devices, and other information storage devices. However, it becomes increasingly difficult to squeeze more information into the storage devices. Moreover, conventional technologies to make those storage devices may be approaching fundamental limits on storage density.
There are many proposed alternatives to increase the storage density of storage devices. Some examples are Scanned Probe Microscopy (SPM), Atomic Force Microscopy, Scanning Tunneling Microscopy (STM), Near-Field Scanning Optical Microscopy, and Scanning Force Microscopy. Each of these proposed alternatives has its own benefits and detriments. Some are extremely expensive to build; some are difficult to implement; others have limited or poor resolution and bandwidth; still others have poor signal-to-noise ratios.
Even if one is successful in increasing the storage density, another major hurdle must still be overcome. Specifically, the time required to access the stored information must be small. Simply put, a storage device's utility is limited if it takes too long to retrieve the stored information, no matter what it's storage density. In other words, in addition to high storage density, one must find a way to quickly access the information.
In U.S. Pat. No. 5,557,596 to Gibson et al., an ultra-high density storage device which provides increased storage density while having fast access times and high data rates is described and claimed. The ultra-density storage device of Gibson et al. is based on the use of electron emitters, which are made by standard semiconductor fabrication technology, and which emit beams of electrons from very sharp points. In one embodiment of Gibson et al., the storage device includes many electron emitters, a storage medium and a micro mover. The storage medium has many storage areas, and the electron emitters are spaced apart to have one emitter responsible for a number of storage areas on the storage medium. In one embodiment, each storage area is responsible for one bit of data on the storage device. The medium is in close proximity to the electron emitters, such as a few hundredths of a micrometer to a few micrometers away.
Each field emitter generates an electron beam current. Each storage area can be in one of a few different states. In one embodiment, binary information is stored in the areas, with one state representing a high bit and another state representing a low bit. When an electron beam current bombards a storage area, a signal current is generated. The magnitude of the signal currents depends on the state of that storage area. Thus, information stored in the area can be read by measuring the magnitude of the signal current. Information can be written onto the storage areas using the electron beams. The magnitude of each electron beam can be increased to a pre-selected level to change the states of the storage area on which it impinges. By changing the state of a storage area, information is written onto it.
Like the electron emitters, the micro mover is made by semiconductor micro fabrication techniques. The micro mover scans the storage medium with respect to the electron emitters or vice versa. Thus, each emitter can access information from a number of storage areas on the storage medium. With hundreds or thousands of electron emitters reading and/or writing information in parallel, the storage device has very fast access times and data rates.
To assure that the storage medium is accurately written to and read as it is moved by the micro movers, it is desirable for the storage medium to have complete ease of motion in the plane of the storage medium, and to have no motion in the direction normal to the plane of the storage medium. In this manner, the distance between the electron emitters and the storage medium is kept constant.
Although it is desired that the storage medium move only in a single plane, achieving this result is difficult. For example, the micro mover is intended to move the storage medium solely in the plane of the storage medium (i.e., in the X-Y plane). However, depending upon the type of micro mover, there may be a tendency for the micro mover to move the storage medium out-of-plane (i.e., displace the storage medium in the Z direction). Also, environmental factors such as vibration may cause or contribute to out-of-plane movement.
Out-of-plane movement may be restricted by decreasing the out-of-plane compliance. Compliance refers to the ease with which the storage medium moves, with higher compliance meaning less resistance to movement. However, restricting the out-of-plane movement of the storage medium by decreasing the out-of-plane compliance often results in decreased in-plane compliance. This decreased in-plane compliance may be sufficient to adversely affect the operation of the memory device because the forces generated by the micro mover are typically not very strong. It thus becomes a balance to suspend the storage medium in a manner which makes the ratio of in-plane to out-of-plane compliance (the “compliance ratio”) as high as possible. A suspension system design which increases the in-plane compliance while maintaining, or improving, the compliance ratio is desirable.